AVFoundation: Call stop() on VideoSurface when changing sources

Before we were reusing the active surface when we switched videos.  This
leads to an inconsitency in states, and the sourceRect property of the
video surface does not get updated.

Task-number: QTBUG-28655
Change-Id: Ie29bf1d9b1c11b6f51a869253c730202001c07cf
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>